a.php

<?php


class GetPut{

    private $values = [];

    public function getVal($key, $timestamp = null){
        if($timestamp !== null && !empty($key)){
            foreach($this->values[$key] as $value){
                if($timestamp === $value['time']){
                    return $value['value'];
                }else{
                    $this->err('timestamp doesn\'t match');
                }
            }
        }else if($timestamp === null && !empty($key)){
            $arrLen = count($this->values[$key]);
            return $this->values[$key][$arrLen - 1]['value'];
        }else{
            $this->err('outside foreach');
        }
    }

    public function putVal($key, $val){

        if(empty($key) || empty($val)){
            $this->err('value missing');
        }

        $timestamp = time();

        if(isset($this->values[$key])){
            array_push($this->values[$key], ['value' => $val, 'time' => $timestamp]);
        }else{
            $this->values[$key][] = ['value' => $val, 'time' => $timestamp];
        }
        echo 'Put value ' . $val . ' at ' . $timestamp . PHP_EOL;
        return [$key, $val, $timestamp];
    }

    private function err($msg){
        echo 'An error has occured ' . $msg . PHP_EOL;
    }
}